1. Conventional Television System
The conventional television broadcast system comprises a transmitting station for providing a composite video signal (which includes a video information signal with standard synchronization pulses), at least one receiver set for receiving the composite video signal, and a medium through which the composite video signal travels from the transmitting station to the receiver set. When the composite video signal comprises a radio wave, the medium is the air through which the radio wave propagates. When the composite video signal is transferred from the transmitting station to receiver set by an electrical signal, the medium may be a cable. Regardless of the medium, however, the transmitting station and receiver set provide similar respective functions.
The transmitting station includes a camera tube having a substantially rectangular raster onto which light from an object, the image of which is to be communicated, is reflected. The raster comprises a photoelectric surface which becomes positively charged as a function of the amount of light which strikes the raster. In order to convert the image on the raster to a signal which can be communicated, an electron beam scans the raster from one area, or element, to another. The scanning electron beam causes adjacent elements on the raster to totally discharge, the discharging of successive elements being directed onto a single line as a discharging current. The discharging current can be changed to RF form by modulating an RF carrier with the discharging current or can be maintained as an IF signal. In either case, the RF or IF signal represents a video information signal which, at any given instant, provides an indication of the amount of light striking (or discharge from) an element of the raster.
In order to effect the desired scanning of the camera tube raster by the electron beam, a horizontal sweep generator and a vertical sweep generator are provided. The horizontal sweep generator causes the electron beam to move in a substantially horizontal trace from the left edge of the raster to the right. Simultaneously, the vertical sweep generator causes the beam to move slowly downward along the raster. When the electron beam reaches the right edge of the raster, the electron beam is returned by a flywheel circuit to the left edge slightly below the line previously scanned. During this return or "retracing", the electron beam is switched off or blanked, no discharge of the raster occurring at that time. Similarly, following a vertical sweep, the electron beam is returned to the upper edge by a vertical retrace during which the electron beam is again blanked. As the electron beam moves from element to element, the amount of discharge of successive elements is indicated by the discharge current and the video information signal. As the beam scans from one element to another along successive horizontal lines, the video information signal provides an orderly, continuous representation of the amount of light striking the elements as the elements are discharged by the electron beam.
At the receiver set of the system is a picture tube having components similar to those of the camera tube at the transmitting station. However, unlike the camera tube which converts an optical image to a transmittable electrical or radio wave video information signal, the picture tube derives an optical image from the video information signal. The picture tube has a screen or raster onto which an electron beam is focused. The intensity of the electron beam is modulated by the video information signal. As the electron beam strikes an element of the picture tube raster, the element illuminates as a function of the intensity of the electron beam. In order to reproduce the camera tube image onto the picture tube raster, the electron beam of the picture tube must scan in a manner similar to the electron beam of the camera tube. That is, the two electron beams must scan the camera tube raster and the picture tube raster, respectively, in synchronism. A horizontal sweep generator and a vertical sweep generator are thus provided in the receiver set, in order to sweep the modulated electron beam across the picture tube raster in a sequence of horizontal scans corresponding to the horizontal scans of the electron beams of the camera tube.
Each scanning of all the distinct horizontal lines of a raster, to provide an instantaneous image, is referred to as a frame. Over the years, the frame has been separated into two interleaved fields. That is, the electron beams will first scan only the odd-numbered horizontal lines proceeding from the top of the raster to the bottom. The electron beams will then be returned to the top of the raster and will then scan only the even-numbered horizontal lines. In interleaving the two fields, it is customary for one of the fields to begin at the top of the raster midway across the horizontal length of the raster, the other field starting at the left edge of the raster. The field starting at the top with a horizontal half-line will terminate at the bottom of the raster at the right edge of the raster, while the field beginning at the left edge of the raster will terminate with a half-line extending from the left edge to the middle of the raster at the bottom of the field. Because of the half-line scanning of the top and of the bottom of the respective fileds, horizontal scans are often referred to in terms of half-lines. That is, instead of characterizing the United States standard as having 525 lines (less the retrace lines), the standard is often described as having 1,050 half-lines.
In order to synchronize the scanning of the electron beam in the picture tube with the electron beam in the camera tube, the transmitting station provides horizontal synchronization (sync) pulses and vertical synchronization (sync) pulses. The horizontal sync pulses are timed to occur at the end of each horizontal scan, while the vertical sync pulses are provided after the beam has reached the bottom of the raster and is in the process of retracing in the vertical direction. The frequency of the horizontal sync pulses and the vertical sync pulses may vary from one video system to another. In the United States and elsewhere in North America, the raster is scanned along 525 distinct horizontal lines (or 1,050 half-lines). Horizontal pulses in the United States are generated at 15,734 Hz with vertical sync pulses being generated at 59.94 Hz. The convention in England is 405 horizontal lines. The convention in Western Europe and Russia is 625 horizontal lines and the convention in France and Belgium is 819 horizontal lines. The horizontal and vertical sync pulses may vary accordingly, the general principles of synchronization still applying.
The horizontal and vertical sync pulses from the transmitting station are combined with the video information signals to form a composite video signal. It is this composite video signal which the conventional receiver set receives. A vital operation of the receiver set is, then, to separate the composite video signal into its components, so that the video information signal may be applied to modulate the electron beam of the picture tube and so that the sync pulses may be separately applied to the two sweep generators of the receiver set, respectively. In order to facilitate the stripping of the sync pulses from the video information signals, the horizontal and vertical sync pulses have amplitudes greater than the maximum possible video information signal amplitude. Generally, the maximum video information signal is 75% to 80% the amplitude of the sync pulses. As discussed below with reference to FIGS. 1 and 2, the separation of the composite video signal includes two steps: first, the stripping of both the horizontal and vertical sync pulses (and other related pulses) from the composite video signal and, second, the separating of horizontal sync pulses from vertical sync pulses.
The waveform of FIG. 1 is a composite video signal generated by modulating an RF carrier with the sync pulses and the video information signals emanating from the transmitting station. An examination of FIG. 1 shows that between times t.sub.1 and t.sub.2, times t.sub.4 and t.sub.5, and times t.sub.7 and t.sub.8, corresponding horizontal sync pulses are provided. The horizontal sync pulses are, as previously discussed, greater in magnitude than the maximum video information signal which can be transmitted. The portion of the waveform between times t.sub.2 and t.sub.3 (and also between times t.sub.5 and t.sub.6) is referred to as the back porch. Carried on the back porch is a signal burst which generally carries color information. Between times t.sub.3 and t.sub.4 (and also between times t.sub.6 and t.sub.7), a horizontal scan is effected. At time t.sub.3, for example, the electron beam is at the left edge of the raster, the amplitude of the RF envelope at time t.sub.3 corresponding to the level of discharge at the leftmost element on the raster. From time t.sub.3 to time t.sub.4, the electron beam scans across a horizontal line to the right edge of the raster. Between times t.sub.4 and t.sub.6, the electron beam returns to the left edge. At time t.sub.6, the electron beam begins scanning a new line. Without the RF carrier applied to the wavefrom, an intermediate frequency (IF) signal is provided which shows only the modulations of the RF carrier and not the carrier itself. At no time is the video information signal amplitude more than 80% of the amplitude of the horizontal sync pulses. Although not shown, a number of pulses are provided at the end of a vertical sweep, these pulses being used to achieve vertical sweep synchronization. These pulses, like the horizontal sync pulses, are also of a magnitude which exceeds the maximum value of the video information signals. By employing conventional clipping circuitry, the receiver set can derive the various sync pulses from the composite video signal of FIG. 1 and can provide synchronization waveforms such as those shown in FIG. 2.
Referring to the upper waveform of FIG. 2, synchronization pulses relating to a first field are shown. The waveform immediately below shows synchronization pulses for a second field which, it should be evident, may be a continuation of the upper waveform. These two waveforms show three types of pulses of differing pulsewidth. First, horizontal sync pulses (at half-line times 18, 20, 22, 24, 544, 546 and 548) are shown having a narrow pulsewidth. These pulses occur during corresponding horizontal blanking intervals. Second, at the beginning and end of each vertical blanking interval, a number of very narrow pulsewidth equalizing pulses are provided at half-line times 0-5, 12-17, 525-530 and 537-542. Third, in the middle of each vertical blanking interval extending between half-line times 0-20 and 525-545 are pulses, each having a wide pulsewidth relative to the horizontal pulses and the equalizing pulses. The three types of pulses in the synchronization waveform are generally applied separately to a differentiation circuit which provides an output of positive and negative spike pulses and to an integration circuit used to provide a vertical sync output that occurs at a relatively constant time in each vertical blanking interval. That is, the integration circuit adds the successive vertical pulses and provides the vertical sync output when a predetermined sum is reached. In the conventional system, the vertical sync output, although occurring at a relatively constant time in each vertical blanking interval, does vary from frame to frame as the vertical pulses in a particular vertical blanking interval may vary in amplitude or duration. Such variations in the vertical sync output are generally of little significance in the conventional system, the vertical sync output providing an input to only a vertical flyback circuit.
In the early prior art, considerable technology has been generated with the aim of tying a transmitting station to one or a plurality of receiver sets and of synchronizing the sweeping of electron beams in the camera tube and picture tube, respectively. For many years in the past, the prior technology has emphasized the importance of synchronization. The prior technology has also been concerned with the separation of vertical and horizontal synchronization pulses from a composite video signal. In sum, the prior art has for the most part been directed to linking a receiver set with a transmitting station and assuring that the transmitting station and receiver set are continuously synchronized.